Throughout this disclosure, various technical and patent publications are referenced to more fully describe the state of the art to which this invention pertains. These publications are incorporated by reference, in their entirety, into this application.
Methods have been developed as lower-cost alternatives to photolithography, the ‘gold standard’ for microfabrication and microfluidic device creation. Duffy et al. first introduced ‘rapid prototyping of masters’ whereby they used printed transparencies to replace the expensive chrome masks traditionally utilized in photolithography (Duffy D., et al. (1998) Anal Chem. 70: 4974-4984). The authors demonstrated the advantages of using rapid prototyping for masks over conventional photolithography and micromachining. Despite its convenience, the method still requires the use of expensive photoresist, high-resolution printing, and contact lithography. Tan et al. obviated this issue by direct printing; they photocopied designs onto transparencies to fabricate microfluidic channel molds that ranged in height from 8-14 micrometer, depending on the darkness setting of the photocopy machine (Tan A., et al. (2001) Lab Chip 1: 7-9). Liu et al. developed a one-step direct-printing technique for the design and fabrication of passive micro-mixers in microfluidic devices, with a maximum channel height of 11 micrometer (Liu A., et al. (2005) Lab Chip 5: 974-978). Such shallow channels are adequate for many microfluidic applications but not amenable for use with large mammalian cells (>10 micrometer in diameter) as well as other applications, such as flowing chemotactic gradients across adherent cells in a channel with minimal shearing (Lin F., et al. (2004) Biochem. And Biophys. Res. Commun. 319: 576-581).
While Lago et al. introduced a way to circumvent the height limitation of single-layer ink by printing up to four times using a thermal toner transfer method onto a glass substrate, the maximum height obtained with this approach was 25 micrometer (Lago C. L., et al. (2004) Electrophoresis 25: 3825-3831). Vullev et al. demonstrated a non-lithographic fabrication approach of microfluidic devices by printing positive-relief masters with a laser-jet printer for detecting bacterial spores; the height of the channels, which is likewise dependent on the height of the ink, is limited to between 5-9 micrometer (Vullev V., et al. (2006) J. Am. Chem. Soc. 128: 16062-16072). To achieve deep channels, McDonald et al. introduced the use of solid object printing (SOP) to make PDMS molds in thermoplastics (McDonald J. C., et al. (2002) Anal. Chem. 74: 1537-1545). However, despite their versatility, solid object printers are considerably costly ($50,000).
Furthermore, the majority of these methods (as well as conventional photolithography) produce rectangular cross section channels. Pneumatic valves, first introduced by Quake et al., important for many microfluidic applications, require microfluidic channels to be rounded such that they can be completely sealed upon valve closure (Unger M. A., et al. (2000) Science 288(5463): 113-116). Achieving rounded micro fluidic channels using typical photolithographic techniques, however, is complicated and requires an extra re-flow step of the photoresist at high temperatures. Most recently, Chao et al. demonstrated an elegant rapid prototyping approach, coined microscale plasma templating (μPLAT), using water molds. This technique enables the creation of rounded channels that are difficult to make with photolithography, but still requires micromachined masks and plasma activation (Chao S. H., et al. (2007) Lab Chip Technical Note 7: 641-643).